In a new study, researchers from the Indian Institute of Technology (IIT) Gandhinagar and the Indian Institute of Science (IISc), Bengaluru, have demonstrated the functioning of a motor protein responsible for recycling material inside a cell. These findings are crucial since faulty recycling in cells is responsible for many disorders including Alzheimer’s disease, Huntington’s disease, and Cancer.
Animal cells take in extracellular materials, a few proteins and lipids by a process called Endocytosis. Some of the endocytosed components are recycled back to the plasma membrane by a process known as recycling endocytosis. The material taken in is sorted within sorting endosomes; whatever is destined for recycling to the plasma membrane is then diverted into a tubular membranous network called the Tubular Recycling Endosomes (TRE).
Motor proteins play a key role in recycling endocytosis as they generate forces to lift the cargo and carry them to the cell periphery along pre-designated lanes of microtubules. The current study shows the mechanism of functioning of one such motor protein — KIF13A, belonging to the kinesin‑3 family.
Subba Rao Gangi Setty, Associate Professor, Department of Microbiology and Cell Biology at IISc and one of the authors of the study says, “We discovered cargo transport mechanisms between endosomes and the plasma membrane in a cell, which is mediated by TREs. Our study illustrates how a family of enzymes called guanosine triphosphatases (GTPases) regulate the activity of KIF13A, which in turn controls what happens to the TREs. This was the key discovery in the study.”
KIF13A, acts in pairs to ferry TREs along microtubules. Wild-type (WT) KIF13A (or when not bound to cargo) exists as a single, inactive molecule. A previous study by the same group showed that the amino acid Proline at the neck of the KIF13A protein introduces a kink in the protein structure that hinders interaction with another KIF13A monomer. How then do two KIF13A monomers pair up to transport TREs along microtubules, has been an unanswered question for some time now.
The researchers, led by Virupakshi Soppina of IIT Gandhinagar, reveal that a protein called Rab22A, a GTPase, binds to the neck region of two inactive KIF13A monomers to activate them — ”like a zipper.” The researchers performed cloning and transfection experiments on cultured cells, followed by live cell imaging, direct fluorescence spectroscopy and in vitro single-molecule reconstitution assays to study the interaction between KIF13A and Rab22A. They found out that binding of Rab22A relieves the kink in the structure induced by Proline, and this opens up the KIF13A structure to pair up. If two inactive KIF13A monomers are akin to either sides of a zipper when it is open, the Rab22A can be compared to the slider that joins both sides and helps them pair up. Rab22A is the molecular switch that controls the two states of KIF13A — active and inactive.
Krishanu Ray, professor at Tata Institute of Fundamental Research, Mumbai, who was not involved in this study, sheds light on the importance of these findings, “ Sorting of proteins, lipids and other metabolites inside a cell is essential for life. This study improves our understanding of a basic cell biology concept underlying development, cancer, and neurobiology. With this knowledge, one can now work at rectifying problems in recycling and correct the disorder.”
The enormity of the findings is further heightened when one realises the challenges associated with such a study. Ray says, “The first challenge is to make motor proteins work when and where you need them to. Secondly, a cell repurposes one type of motor for multiple functions. Hence, it can be an uphill task to demonstrate a specific action played by Rab22A at a specific time point. I consider this study as an exquisite demonstration of a precise cell biology experiment.”
The authors believe that this study is the first of its kind to report a Rab protein regulating activity of a motor protein to facilitate intracellular transport. This knowledge is vital to further understand diseases arising from faulty Rab-motor protein interactions. Additionally, these findings are highly relevant in the current times —KIF13A has been known to play a role in the trafficking of viral particles. Understanding its regulation can be a step forward in the prevention or treatment of viral diseases.